This invention relates to the stabilization of vinyl chloride polymers. More particularly, it relates to such stabilization as results from the action of a combination of additives. Still more particularly, it relates to certain vinyl chloride compositions which are notably stable at high temperatures and which, also, are relatively stable to coloration on exposure to ultraviolet light.
A very difficult problem which faced early processors of vinyl chloride polymers was that of stability. It had been known that these polymers are heat sensitive, light sensitive and subject to the corrosive action of metals, particularly iron. Since it is not possible to process the material at ordinary temperatures, or to use a non-ferrous paint, and since a light-sensitive material is of no use for the fabrication of consumer goods, all of these problems had to be solved if the art was to be developed.
Very large amounts of vinyl chloride polymers are produced today, largely because of their great versatility. This versatility, however, exacerbates the problem of stability. Means must be found of stabilizing such polymers under conditions ranging from the extrusion of inplasticized polymer at 200.degree. C. to the calendering of material containing up to 50% of plasticizer at 190.degree. C., or of stabilizing either type of composition so that it will withstand a molding process with a long cycle. The finished product may have to be crystal clear, or it may be loaded with heavy fillers. It may be processed hot, or applied as a cold, free-flowing paste, and then heated rapidly to a high temperature. There is, thus , a need for a wide variety of stabilizers.
It is known that vinyl chloride polymers undergo chemical degradation and darkening when heated under conditions of molding or extrusion. Many types of chemical stabilizers are effective to inhibit these types of deterioration, including various metal salts of inorganic and organic acids and certain organometallic compounds, especially organotin compounds.
Hechenbleikner (U.S. Pat. No. 3,205,250) suggests the use of dialkyl pentaerythritol diphosphites as stabilizers for polyvinyl chloride.
The "Encyclopedia of PVC," Vol. 1, edited by Nass and published by Marcel Dekker, Inc., New York (1976), pages 308-9, suggests the use of mannitol, sorbitol, etc., as stabilizers for polyvinyl chloride. Phosphites are also suggested for use in polyvinyl chloride.